1. Field of the Invention
The present invention relates to an ethylene-based polymer resin excellent in balance of extrusion moldability, mechanical strength and appearance of extruded molded article.
2. Description of Related Art
Ethylene-based polymer resins have been used in a variety of fields as a general purpose type resin, and are required to have mechanical properties such as, for example, impact strength, appearance of extruded articles such as film and sheet, such as surface smoothness, gloss and transparency, in addition to extrusion moldability such as, for example, extrusion torque, melt tension and high speed processability. A low density polyethylene obtained by a high pressure radical polymerization method has defects that molding at high speed is difficult because of too high melt tension though balance between extrusion torque and melt tension is excellent, further, that impact strength is low. On the other hand, regarding a conventional linear low density polyethylene using an α-olefin such as 1-butene or 1-hexene as a copolymerization component with ethylene, methods of increasing molecular weight and decreasing density for improving mechanical strength and methods of increasing molecular weight for improving melt tension, are envisaged, however, these are not suitable methods since deterioration in moldability due to increase in extrusion torque during molding and deterioration in rigidity and heat resistance, is invited.
JP-A-04-213309 discloses a polyethylene copolymer having an extremely high melt tension, however, it has defects that high speed hauling off is difficult due to too high melt tension and consequently molding speed is restricted, therefore, it can not be said that requirements are satisfied. JP-A-06-9724 discloses an ethylene copolymer having relatively low melt tension, however, it has a narrow molecular weight distribution and its flowability is not admitted as sufficiently excellent, and also in relation between melt index and intrinsic viscosity, it is rather nearer to conventional linear low density polyethylenes than high pressure processed low density polyethylenes excellent in processability, not sufficiently satisfying conventional requirements in moldability. Therefore, it is extremely difficult to enable even molding at high speed with improving mechanical strength and melt tension while suppressing increase in extrusion torque during molding and decrease in rigidity, and development of such resins is strongly desired. Further, molded articles such as films obtained from such resins not only manifest extremely easy molding but also show excellent mechanical properties and appearance such as surface smoothness, gloss and transparency of extruded articles such as film and sheet, therefore, practical utilization thereof is strongly desired.